Is this on the blog post, or are you trying it out yourself?

If it's on the blog post, it sounds like your browser doesn't support `SharedArrayBuffer` (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...) or you've disabled Cross Origin Isolation somehow. It might be because you have an old browser (there was a period where it was disabled) or you have disabled Cross Origin Isolation yourself (maybe a security setting?).

SharedArrayBuffer was affected by Spectre vuln and was disabled in some old browsers (or browser config).